A handsfree communication apparatus, for example a handsfree telephone is typically characterized by transmit and receive channels, a transmission facility interface and a controller for biasing the operation of the apparatus toward one of transmit and receive modes of operation. The controller includes a comparator circuit which compares signals in the transmit and receive channels and determines the mode of operation in accordance with which channel has the strongest signal. The two modes of operation are essentially mutually exclusive in order to prevent oscillation due to feeback between a microphone in the transmit channel and a loudspeaker in the receive channel. Fortunately persons involved in a telephone conversation seldom talk at the same time and therefore virtually normal telephone conversation can be handsfree voice switched if the controller is able to reliably identify which party is talking and which party is listening. A primary problem in handsfree telephony is that of instantaneously determining which of the two modes should be prevalent.
For example, the control mechanism should be sensitive to a user's voice signal received by the microphone to activate the transmit mode. However it should be able to distinguish between the user's voice signal and other voice band frequency signals such as background noise and those signals produced by the loudspeaker, otherwise undesired switching to the transmit mode will occur. Furthermore the control mechanism should also be sensitive to voice signals received from an associated transmission facility, for example a telephone line, to activate the receive mode. However it should be able to distinguish received voice signals from other voice band noise signals received from the telephone line.
All of these requirements have been achieved in the past with some degree of success, sufficient to permit the manufacture and sale of voice switched handsfree telephone apparatus. However the quality of voice switching operation seems to vary directly with complexity and cost of manufacture. For example, a simple control mechanism is one which normally operates the apparatus in the receive mode and only switches to the transmit mode in response to a strong signal at the microphone. If the control mechanism is sensitive enought to avoid first syllable clipping in switching to the transmit mode, the handsfree operation also tends to chatter or oscillate between modes and in higher noise environments will lock out the receive mode. Therefore the control mechanism is typically compromised to avoid persistent chatter or lock-out while introducing some undesirable clipping.
In another more elaborate example of a voice switched handsfree telephone as disclosed by J. L. E. Thompson et al, separate envelope detectors to distinguish between noise-like monotonous signals and voice-like fluctuation signals are provided in a control mechanism which provides an idle mode of operation intermediate the transmit and receive modes of operation. When neither party is speaking the idle mode prevails with both the transmit and receive channels being controlled to be partially ON, and biased more or less toward the receive mode in accordance with there being more or less of the monotonous signal from the microphone. Hence the control mechanism can respond more quickly to a first syllable of speech signals in the transmit and receive channels to avoid first syllable clipping without the consequence of chatter problems. However, in spite of additional circuitry tailored to augment and delay transitions between full transmit and full receive modes for optimized performance, the handsfree voice switched telephone conversation remains uncomfortable for persons not normally used to conversing with someone using the apparatus.